The present invention relates to a shotting turbine allowing a rate of ejection of the shot and a concentration of the jet of shot superior to those of known shotting turbines.
Shotting turbines are used to fling shot onto the surface of a metal part in order to give this surface a desired state of roughness. Turbines of the centrifugal type comprise a central delivery device which provides a supply of shot and a plurality of blades or vanes arranged radially around the delivery device and fixed to one or two flanges. The effective surface of these blades is conventionally plane and of uniform width.
This straight blade arrangement flings the shot at a rate of ejection directly dependent on the rate of rotation of the turbine. In certain cases, this rate can be too low to achieve the purpose desired. The impact of the particle, namely, its energy E=(m V.sup.2.sub.R)/2 where m=the mass of a particle, is, in fact, the function of the square of the rate of ejection V.sub.R.
Certain parts to be shot, for example, rolling-mill rolls, are so hard that the desired degree of roughness cannot be achieved with a straight blade arrangement. Like any machine, the shotting turbine has a limiting speed which cannot be exceeded for reasons of safety and wear among others. The straight blade arrangement does not allow a rate of ejection of the shot to be produced which exceeds that corresponding to the limiting rate of rotation of the turbine. For example, a turbine with straight blades, rotating at a speed of 2,500 revolutions per minute would provide a rate of ejection of shot of approximately 77.77 m/s.
A higher rate of ejection has definite advantages however. In the first place, to obtain the same level of roughness the rate of rotation can be lower. The rate of rotation of the turbine is an important element, since in a shotting turbine the differential wear of the blades and flanges brings about troublesome imbalances (vibrations). As these imbalances are a direct function of the square of the rate of rotation they are reduced at the same time as the latter. It is therefore important to reduce as far as possible these disruptive forces and an extremely effective means is therefore to reduce the rate of rotation of the turbine.
On the other hand, a higher rate of ejection than with the straight blade arrangement enables higher degrees of roughness to be obtained from the same rate of rotation. In the particular case of rolling-mills rolls, for example, the roll hardnesses achieved at the present time cannot be increased, since impossibility of shotting is quickly reached, namely, the impossibility of attaining the required degree of roughness. There is therefore often in this field a compromise between the desired degree of roughness and the roll hardness which is as high as possible but permits the attainment of the required degree of roughness. For example, with angular shot composed of particles having an average size of 0.40 mm and width a roll hardness of 730-750 Vickers hardness under a load of 30 kg (HV), the maximum degree of roughness reached is 200.mu." (CLA: Center Line Average). If the roll hardness is increased by 30 points on the Vickers hardness scale, the maximum possible degree of roughness will be, for example, 170.mu." (CLA).
Merely using curved blades does not solve the problem of improving satisfactorily the rate of ejection of the shot. It has been found, indeed, that the rate of ejection is a function of the friction coefficient of the blade and the angle of curvature thereof, and that using blades with working faces having a convex longitudinal profile only provides a slight increase of the rate of ejection of shot for a small angle value. Increasing the value for said angle of curvature rapidly causes the rate of ejection to decrease as low as a level below the rate of ejection for straight blades.
A first object of this invention is to provide a shotting turbine having blades with working faces having a longitudinal profile optimized so as substantially to improve the rate of ejection of the shot.
Another aspect of this invention relates to the concentration of the jet of shot which is spread out both in the direction of movement of the turbine and in a transverse direction. To take a specific example, with a turbine rotating at a speed of 2,500 revolutions per minute and having straight blades with a uniform width of 60 mm, the spreading of the shot gives at a distance of 500 mm from the turbine a jet having an impact length of approximately 793 mm and an impact width of approximately 80 to 90 mm. It has been established that if the central part of the jet produces a uniform roughness on the surface which it touches, the marginal parts of the spread jet contain particles of shot which rebound on the surface to be shot, which break down and which have an impact effect which is prejudicial to the efficiency of the operation. This spreading of the jet thus appreciably limits the impact force of the jet and it is especially troublesome in the shotting of parts having a curved surface, for example, rolling-mill rolls. The improvement in the transverse concentration of the jet of shot brings about a reduction in the spread of the jet, the effect of which is to increase the impact power of the jet for the same delivery of shot.